The invention relates to a method for stopping a machine reel in a paper machine in the final stage of reeling a web, wherein after the reel change, stopping braking forces are exerted on the centre-drive and on the surface of a rotating machine reel detached from the reeling cylinder. Hereinbelow, a shorter term xe2x80x9creelxe2x80x9d will be used for the machine reel.
It is a known procedure to stop the reel by means of a braking force which is exerted exclusively on the centre-drive, i.e. is effective from the shaft of the reel, and which is opposite to the direction of rotation of the reel spool. The method is applicable especially in situations where a large amount of time has to be used or can be used for stopping. The amount of time to be used is primarily affected by the efficiency requirements of the paper machine and on the other hand by the dimensioning of the space to be used in the change situation, in other words how well and how long the new reel to be stopped and the new reel that is becoming full simultaneously fit in the reel-up, e.g. on the same reeling rails.
Especially along with the growing efficiency requirements, it has become necessary to shorten the stopping time, which easily causes considerable problems when stopping is conducted only by means of a braking force effective from the shaft. Especially in case of slippery paper grades, the act of increasing the braking force to accelerate the stopping causes so-called creeping i.e. xe2x80x9cunrollingxe2x80x9d of the surface layers of the reel, because some surface layers tend to continue their movement in the direction of rotation, i.e. the direction of unrolling of the reel, at the original speed of rotation, irrespective of the decreasing speed of rotation of the inner layers, which results in that the reel is slackened. The slackening takes place on the outer perimeter because the unrolling of the spiral structure of the reel is first possible therein, and, on the other hand, the largest mass and the largest diameter i.e. the largest inertia momentum, is present on the outer perimeter. The unrolled surface section of the reel is unfit for use and causes broke. In an on-line coating and calendering line, the need to discharge broke can grow as high as 2500 meters.
To eliminate the above-described problem, there are known solutions which are based only on the use of braking force exerted on the surface of the reel, and solutions which are based on the use of two separate braking forces simultaneously, i.e. a braking force effective from the shaft and a braking force exerted on the surface of the reel, thus being effective through the surface.
Force effects which are exerted especially and/or solely on the surface of the reel are presented for example in the publications EP-483 092, EP-658 504, U.S. Pat. No. 3,471,097 as well as in the publication FI-95683 and in the corresponding international publication WO 95/34495.
In the two first-mentioned publications, the presented embodiments include a press roll which forms a nip with the surface of the reel, and in the latter of these also a set of belts that forms a nip. In these embodiments, the purpose of the press rolls and the set of belts is primarily to prevent the access of air between the layers of paper when the machine reel is detached from the reeling cylinder and when the web is still passed on the roll, and not to affect the stopping of the roll. Furthermore, the publications do not present solutions for producing a braking force effective from the shaft.
The solution according to the patent publication U.S. Pat. No. 3,471,097 includes a set of belts which is intended to function as a part of the devices performing a so-called bag change. The purpose of the set of belts is also to decelerate the rotation of the reel and to stop it eventually. The braking force is not exerted on the reel in other ways. In the process of stopping the reel in a manner complying with the requirements of present paper machines, the sole use of such a set of belts as an actual braking member would result in an excessive loading force exerted on the surface of the reel, which could easily cause damaging of the surface layers and a growing need to discharge paper as broke, i.e. it would not be advantageous to prevent the surface layers from unrolling.
The purpose of the member disclosed in the Finnish patent publication FI-95683, exerting loadig force on the surface of the reel, is especially to prevent the access of air between the layers of paper when the web is reeled on the reel while the nip between the reel and the reeling cylinder is open. Said member can be a fragmentary roll or a spreader bar, but particularly the publication mentions a brush device by means of which especially a radial force effect is to be exerted on the surface. Even though it is possible to reduce the creeping risk of a full reel by using a brush device or the like, at the same time producing especially radial braking force, there are, however, problems related to the use of this solution with respect to stopping the reel. Even if the aim was to attain significant braking forces by means of this device, considerably large radial forces would be required because the braking force is primarily produced by means of the friction between the press contact surface of the device and the outer surface of the reel, and the friction between the paper layers. Also in this case, the use of large braking forces would easily cause unnecessary damaging of surface layers and a need to discharge paper as broke. Furthermore, because of the surface structure of the brush device, it is not possible to manufacture a practicable brush device intended for braking and stopping in particular. Similarly, it is obvious that said fragmentary roll and spreader bar are not intended for exerting significant braking forces on the reel.
German utility model DE-29604401 discloses an arrangement for braking and stopping the reel in such a way that the reel is decelerated both by means of a braking force effective from the shaft and a braking force produced on the surface of the reel, formed from a radial and peripherally directed (tangential) force, applied from outside to the perimeter of the reel. The braking force effective on the surface is produced by means of a press device, especially by means of a press roll, which is loaded against the reel to attain a radial force, and which is equipped with a drive to accelerate its speed of rotation to the peripheral speed of the reel before it is brought in contact with the reel. The press roll also includes an adjustable brake device to produce and adjust the peripheral braking force affecting the reel. This publication suggests that the braking power exerted on the surface should be greater than the braking power exerted on the shaft, wherein the braking force effective on the surface has to be increased unnecessarily high. This also requires substantially powerful devices to attain the loading effect of the press member as well as to affect the rotation. As the aim of the above-presented solution is to prevent the creeping of the surface layers, it is obvious that great braking forces effective on the surface easily damage the surface layers of the reel. This effect is even stronger if, according to the presented solution, said braking forces have to be increased along with the increased creeping risk.
It is an aim of the present invention to introduce a method for stopping a reel, by means of which method the deficiencies of the above-presented prior art solutions can be eliminated to a large extent, thus improving the state of art prevailing in the field. To attain this purpose, the method according to the invention is primarily characterized in that the braking powers which stop the reel and are exerted on the centre-drive of the reel and on the surface of the reel, i.e. braking powers which act through the shaft and through the surface, are distributed during the stopping according to the ratio of S=1:1 . . . 100:1. Thus, the braking power effective on the surface is at the most equal to the braking power effective from the shaft, and it can be reduced therefrom or changed further, if necessary, during the stopping. The range in which the ratio of the powers lies can also be 1:1 . . . 50:1, 5:4 . . . 100:1, 5:4 . . . 50:1, 3:2 . . . 100:1, 3:2 . . . 50:1, 1:1 . . . 10:1, 5:4 . . . 10:1, 3:2 . . . 10:1, 1:1 . . . 5:1, 5:4 . . . 5:1, or 3:2 . . . 5:1. The ratio can be the same, or it can be changed, preferably in such a way that the ratio remains within the above-defined range most of the braking time, for example at least {fraction (9/10)} of the total braking time, or all the time when braking power is exerted on the reeling shaft and on the surface of the machine reel by means of corresponding brake devices.
By means of the method according to the invention, it is possible to minimize the stopping time of the reel and at the same time to reduce the damaging of the surface layers as well as the need to discharge paper as broke. In the method, attention is paid to the fact that the braking force effective on the surface produces a controlled stopping together with the friction forces of the paper layers, even in such a manner that the braking and stopping take place without the creeping phenomenon in the surface layers and the slackening of the reel. To implement the stopping in an optimal manner it is possible in the method to pay attention during the entire stopping process to the magnitude of all braking forces and braking torques produced as well as to the changes therein, to the weight and speed of rotation of the reel and the changes therein and to the friction properties of the paper. For example on the basis of these variables the most appropriate braking force and/or torque at a given time is changed. Because substantially smaller braking forces than those used in solutions of prior art are necessary when the method according to the invention is applied, it is possible to keep the structures and solutions in a relatively moderate level with respect to the output of the apparatus to be used.
The invention is suitable to be utilized in connection with continuously operating reel-ups in which machine reels are formed from a full-width paper web of several meters in width (e.g. 5 m or more) issuing from a paper machine or a finishing machine for paper. Growing machine speeds, machine widths and maximum diameters of the reels result in reels of large mass that rotate at high speed after the cutting of the web. These reels have to be decelerated as rapidly as before despite of the large rotational energy, or because of the time available, one has to aim at even shorter braking times. In case of a cylindrical body, which is a good approximation of the machine reel, the rotational energy is directly proportional to the mass of the body, the square of the angular speed of the body and to the square of the radius of the body. It can be proven by calculations that when the web speed (the surface speed of the reel) grows by 50%, the mass of the reel by 125% and the diameter of the reel approximately by 25%, the rotational energy grows approximately fivefold, which considerably increases the braking time when the same braking torque is used.
Other features characteristic to the method according to the invention will become apparent from the appendent dependent claims and from the description hereinbelow.